1. Field of the Invention
The invention relates in general to a method of fabricating a contact window, and more particularly, to a method of fabricating a contact window with a chamfered top corner by forming an additional gettering layer and a hard mask layer.
2. Description of the Related Art
Due to the higher and higher integration in an integrated circuit, surface areas on a chip available for forming interconnects become more and more limited. To achieve the electrical connection between electrodes or devices on the chips, a technique of multi-level interconnection was developed and is now commonly used. During the fabrication of a multi-level interconnect, an inter-metal dielectric (IMD) layer is formed between two conductive layers to avoid any unwanted connections. To electrically connect the two conductive or metal layers, a contact window is formed to penetrate through the inter-metal dielectric layer. The contact window is then filled with a conductive layer to form contact via or a contact plug. Another conductive layer is further formed and coupled with the contact via or contact plug. Thus, two conductive layers can be electrically connected with each other by means of the contact via or contact plug.
In FIG. 1A to FIG. 1C, a conventional method of forming a contact window is shown.
In FIG. 1A, an inter-metal dielectric layer 14, for example, a silicon oxide layer formed by chemical vapour deposition (CVD), is formed on a semiconductor substrate which includes a conductive region 12 such as a source/drain region, a gate, or other conductive structures. A photo-resist layer 16 is formed on the dielectric layer 14 with an opening aligned with the conductive region 12. That is, the opening exposes the dielectric layer 14 right on top of the conductive region 12.
In FIG. 1B using a dry etch, the exposed dielectric layer 14 is removed with the photo-resist layer 16 as a mask. A contact window 18 is formed on the modified dielectric layer 14a to expose the conductive region 12.
In FIG. 1C, using sputtering deposition, a glue/barrier layer 20 is formed of, for example, titanium/titanium nitride (Ti/TiN), to enhance the adhesion between a contact via formed subsequently and the contact window 18.
As shown in FIG. 1C, by using this conventional method, a sharp corner 22 is formed on the top edge of the contact window 18. With the sharp corner 22, a poor step coverage is obtained for forming the glue/barrier layer 20. As a consequence, an overhang of the glue/barrier layer 20 is formed at the top corner 22 of the contact window 18. In the subsequent process of forming a contact via or a contact plug, the overhang causes the formation of a void which causes a poor conductivity. A bad contact is caused between the conductive layer 12 and another conductive layer formed thereafter. Therefore the contact via or contact plug cannot form a good electrical connection between the conductive layer 12 and another conductive layer.
Moreover, during the dry etch process to remove the inter-metal dielectric layer 14 and the deposition process for different layers mentioned above, moisture and alkaline metal ions are existent. The moisture and alkaline metal ions easily penetrate through the dielectric layer 14. and therefore, the structures or components under the dielectric layer are damaged and degraded by the penetrated moisture or alkaline metal ions. The quality of the integrated circuit is degraded and the lifetime of the integrated circuit is shortened. Furthermore, during the exposure process for defining the contact window 18, without the formation an anti-reflective layer, an incident light source transmits through the inter-metal dielectric layer 14 and is then reflected by the conductive layer 12 or other metal layers underlying the inter-metal dielectric layer. The reflective light interferes with the incident light source, so that a misalignment is easily caused. Sometimes, the misalignment even causes a blind window. The blind window occurs when photo-resist layer or the inter-metal dielectric layer is not removed completely from the contact window. T he conductive layer within the contact window is thus not exposed at all, and the electrical connection cannot be achieved by the formation of the contact via or contact plug. It is known that the thickness of the dielectric layer is a key factor which affects the critical dimension (CD) in after develop inspection (ADI). If a deviation is introduced due to defocusing or a swing effect of the dielectric layer, an error of critical dimension occurs. In the conventional method, if the photo-resist layer is not thick enough, it is possible that the photo-resist layer is removed while removing the exposed dielectric to form the contact window. Therefore, to be used as a mask layer, the photo-resist layer has to be sufficiently thick to protect the underlying dielectric layer from being etched or removed. However, the thicker the photo-resist layer is, the smaller the tolerance of critical dimension is. Thus, a defocusing phenomenon occurs often.